The use and construction of optical fiber cables, and of optical fiber build-outs or adapters used for coupling optical fiber cables together is well known. The optical fiber cables to be coupled are typically provided with optical fiber connectors at their ends, the connectors being one of the several known types of connectors, to include, for example, one or a combination of the SC, an ST.RTM., an FC and/or LC types of optical fiber connectors. The build-out or adapter will typically comprise a housing constructed for receiving two opposed optical fiber connectors therein so that the ends of the opposed ferrules housing the optical fibers are axially aligned for transmitting an optical signal therethrough and along a transmission path extending the length of the connected optical fiber cables.
It is oftentimes desirable, if not necessary, to introduce an attenuator or some type of attenuation device into the transmission path of the optical signal in order to reduce the strength of the incoming signal to a desirable or manageable power level. Such attenuation oftentimes occurs at a patch panel or bulkhead where the attenuating element remains in a fixed position with respect to, and is not otherwise a part of or incorporated into the optical fiber. Accordingly, the attenuation element or device is typically incorporated into the coupling, also referred to by those skilled in the art as a build-out or an adapter, which extends through the panel and into which the incoming and outgoing optical fiber connectors and cables are attached, respectively.
The ability to change the attenuation level is also needed, however, as it is known that the efficiency of an optical fiber circuit decreases with age, in that the power of a signal source, which may be adequate at the beginning of the life of the circuit, may later become inadequate. Moreover, if the power of the signal at the beginning of the service life of the circuit is chosen so that it remains adequate as the circuit ages, other components of the circuit may become saturated early in the circuit's life, rendering them less efficient. In addition, the unearthing of optical fiber cables, for example by being accidentally cut or damaged during construction or installation, results in repairs to the cable that add optical loss to the transmission path which also requires that any attenuation element(s) in use be decreased in strength in order to make up for the loss in signal strength caused by any such cable splices.
The known types of optical attenuators generally fall into four categories. First, there are air gap attenuators in which various fixed filter elements are suspended in an air gap between the ferrules of the optical fiber connectors, the ferrules being received in a coupling or alignment sleeve which includes a means for preventing contact between the two ferrule ends, or with various filter elements suspended in the area. A second category of attenuators comprise an air gap in which attenuation is increased by increasing the length of the gap, i.e., the distance between the opposed ferrule ends within the alignment sleeve. In a third type of attenuator arrangement, a high density, translucent, laminated attenuator element that may be varied in the thickness is provided for different attenuation levels, and is mounted in a transverse slot within the alignment sleeve. A fourth class of attenuators are those used with biconic connectors of the type having two tapered plugs, with the plugs being received in a sleeve having opposed conically shaped plug receiving cavities.
One of the more common attenuator arrangements currently used is the third type of attenuator arrangement described above, in which a plate like element is suspended or positioned between the opposed ferrule ends of two optical fiber connectors, and can be varied in thickness for attenuating the optical signal passed therethrough and along the transmission path. Examples of such prior art attenuators are shown in U.S. Pat. No. 5,067,783 to Lampert, and U.S. Pat. No. 5,082,345 to Cammons, et al., both of which are assigned to Lucent Technologies, Inc. This type of prior art attenuator element is also illustrated in FIG. 1, and is described in greater detail below.
Referring now to FIG. 1, an attenuator element 5 is shown positioned within an elongate alignment sleeve 7 of an optical fiber adapter (not illustrated), and is formed about a longitudinal axis denoted by the reference character "A.sub.x." A ferrule 8 of a first optical fiber connector (not illustrated) is spaced from and opposed to an axially aligned ferrule 9 of a second optical fiber connector (not illustrated) along the longitudinal axis A.sub.x of the alignment sleeve. The attenuator element comprises a cylindrical disk 11 positioned between the optical fiber 12 of ferrule 8, and the optical fiber 13 of ferrule 9, and is used to decrease the signal strength of the optical signal passed along the transmission path comprised of the two optical fibers.
The attenuator disk is positioned between the two optical fibers by being suspended from a shoulder 15, the shoulder depending from a head 16 received within an elongate slot 17 formed in the alignment sleeve. The slot 17 extends in the lengthwise axial direction of the sleeve. As shown in FIG. 1, the ends of the ferrules 8, 9 are formed to be slightly spherical, with a complimentary spherical curve being embossed or otherwise formed in the attenuator disk 11 such that the ferrules are received snugly against the attenuator disk, but are spaced from one another such that the ferrule ends do not touch each other within the alignment sleeve 7.
This type of attenuation arrangement has proven to be highly successful, and is commonly found in use with a wide variety of optical fiber adapters. However, although this type of attenuator element/device has proven to be highly successful, improvements in the coupling of optical fibers together, through the use of spherical angled ferrule end faces, provides a construction for which this type of attenuator element construction is not well suited. Examples of the newer angled spherical construction ferrule end faces are disclosed in U.S. Pat. No. 5,384,885 to Diner, and in U.S. Pat. No. 5,351,327 to Lurie, et al.
In the '327 patent to Lurie, et al., a method of polishing optical fiber ferrules which centers the apex of the spherical end face of the ferrule on an angled optical fiber axis is disclosed. The patent to Lurie, et al., teaches that signal transmission across optical fiber connectors may be improved by imparting an angled finish to the end face of the fibers, i.e., the ferrules, which results in reduced internal reflections of the signals at their end faces. This is so in that in virtually all coupling arrangements in which optical fibers are coupled, the coupling by its very nature introduces discontinuities in the transmission path, i.e., signal reflections, which constitute a serious problem. In simple coupling arrangements, of the type described in the '783 and '345 patents above, these reflections exist, and where an attenuating element such as that shown is present, the problem of reflection may be magnified in that incident power, the amount of light intentionally transmitted from the source to the receiver along the transmission path, may be reduced by unwanted increases in reflected power. As known to those in the art, reflected power describes that amount of light which is reflected at an interface, for example an attenuation element, or other discontinuity in the optical fiber cable, which light reflection can travel back through the fiber toward the light or signal source so that the light or signal source is adversely affected in its performance.
In an effort to combat these problems, the '885 patent to Diner provided a means for attenuating an optical signal transmitted through angled ferrule ends by providing a variable attenuation optical fiber coupling arrangement. In Diner the end faces of each of the optical fiber connectors (ferrules) are formed at an angle with respect to the optical fiber axis, each connector having a locating key thereon so that the connectors may be selectively rotated in position with respect to one another, relative to the fiber end faces, so as to introduce a measured amount of misalignment of the end face of the fiber relative to the other fiber end face for "tuning," as it were, the optical signal passed therethrough and along the transmission path.
Although the use of angled spherical ferrule end faces provides for improved incident power levels and reduced reflections at optical fiber couplings, the large majority of optical fiber couplings are still formed along a longitudinal axis such as that described in the '783 and '345 patents to Lampert and Cammons, et al, respectively, and as illustrated in FIG. 1. However, as the use of angled ferrule end faces increases, due to the improved performance characteristics of such a ferrule construction, the need arises for a simple type of attenuation element which can be used not only with longitudinally aligned optical fiber connectors, but which may also be used with the new generation of "angled" optical fiber ferrule end faces so that only a single type of attenuation device/element need be provided for use with any of the known types of optical fiber couplings without concern as to whether the ferrule end faces are to be longitudinally aligned or angled with respect to one another. None of the known types of attenuator elements, or devices, however, appear suitable for accomplishing this task.
The need exists, therefore, for an improved attenuator element which is capable of being used with any type of optical fiber coupling, and in particular with ferrules having longitudinally aligned spherical end faces, as well as ferrules having angled spherical end faces in any of the several known types of optical fiber adapters/build-outs.